With the increasing threat of global energy and serious environmental problem, the renewable energy has been developing fast. Photovoltaic industry has an extensive future as its resources are plentiful and widespread. It is an important issue to lower cost and improve efficiency for photovoltaic system.
It is known that photovoltaic arrays yield DC power. Inverter is used to convert DC from photovoltaic arrays into AC in photovoltaic system. There is a common mode loop circuit in the photovoltaic inverter system without a transformer. The high frequency leakage current in the loop circuit leads to electromagnetic interference, and at the same time, it is dangerous to devices and human. So the high frequency leakage current becomes a critical issue to be solved for the transformerless inverter system.
Inverters are divided into two kinds according to the circuit structure.
The first kind of inverter is symmetry in topology and it has double AC filtering inductors. Full bridge inverter circuit is usually of this kind. No additional circuit in the full bridge inverter circuit is needed to boost input voltage in many cases since it is enough for half of that of the half-bridge. But it is difficult for the full bridge inverter to cancel substantially the high frequency leakage current because of the parasitic parameters within the inverter system. The improvements of conventional H4 full bridge circuit are made to reduce the high frequency leakage current so that it meets the industry standard. However, its cost is high because the two AC filtering inductors' magnetic cores are not common for the symmetry mode. In addition, cost is high because of double AC filtering inductors.
The second kind of inverter is non-symmetry and it has single AC filtering inductor. Half bridge and mid-point clamped inverter circuits are examples of this kind. One terminal of AC utilities or AC load is connected with the mid-point of DC input voltage for the single phase inverter. The parasitic capacitor voltage is constant so that the leakage current is eliminated substantially. However, it needs an additional circuit to increase the input voltage as the voltage utilization ratio of the half bridge based inverter (the second kind) is half of that of a full bridge based inverter (the first kind). This additional circuit reduces the system efficiency and increases the system cost.
For the three-phase inverter system, three-level half-bridge inverter is often used. However, its cost is high because (1) the voltage utilization is low, (2) the AC filtering inductance is high and (3) additional circuitry is needed to increase the input DC voltage.